The invention relates to the measurement and adjustment of the power of an RF signal input into a laser used for optical communication, typically over a fiber optic link.
It is important to regulate the RF power of a signal input into a laser transmitter: if the power is too high, distortion results; on the other hand, if the power is too low, noise results. Both distortion and noise lead to degradation of the signal transmitted over the optical communications link (such as a fiber optic cable) and it is therefore desirable to reduce both noise and distortion in the signal.
The reduction of noise and distortion may involve first determining the amount of noise and distortion in a signal, and then reducing these unwanted features. There are a number of ways in which the distortion and noise of a signal may be measured. For example, Third Order Intercept, Carrier to Intermodulation, Spur Free Dynamic Range, Noise Figure and Carrier to Noise Ratio are all methods which may be employed to measure noise and distortion. Noise Power Ratio (NPR) is particularly useful as this is a single measurement combining the effects of both distortion and noise.
A number of methods of regulating the power of a RF signal input to a laser are known. A first known method of regulating the power of the input RF signal to a laser (herein referred to as “SL”) is by using a fixed gain amplifier. However, such a fixed gain amplifier produces a gain for the RF input signal to the laser which may, as the characteristics of the RF input signal vary, result in degraded performance. An amplifier with a fixed gain setting is unable to adapt to changes in the power of the RF input signal. A fixed gain setting which may be optimal initially may therefore result in noise and distortion as time progresses. Furthermore, where the RF input signal for the laser includes more than one data channel multiplexed over a single optical fiber link, with each of the channels corresponding to a different data source, an optimal setting for one of the channels will not necessarily be optimal for any of the other channels, resulting in further potential degradation of the quality of the communication. For these reasons, such fixed gain amplifiers often result in degraded performance.
A second known method of controlling the power of a signal input to a laser involves the use of a manual gain control. Manual gain control suffers from the disadvantage that it requires a degree of skill and expertise on the part of the user to be implemented effectively. Furthermore, if the power of the RF input signal varies significantly over time, further user intervention is required to optimize the gain of the amplifier. Similar to fixed gain control, manual gain control suffers from the disadvantage mentioned above: where the RF input signal to the laser represents multiple data channels multiplexed over a single optical fiber link, the user is unable to easily determine the optimum gain setting to avoid noise and distortion in the RF input signal.
A further known method of optimizing gain control is automatic gain control which suffers from the disadvantage that where multiple channels are transmitted across a single fiber optic communication link, a change in gain in a particular channel may degrade the performance of another channel.
It is also known to combine automatic gain control with fixed gain control by switching between these two modes so that the gain setting for fixed gain control is automatically determined during an initialization step using automatic gain control. However, this requires a certain degree of expertise on the part of the user and can result in errors if not implemented correctly.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for designing other methods and systems for carrying out the several purposes of the present invention. It is important therefore that the claims be regarded as including equivalent constructions insofar as they do not depart from the scope of the present invention.